Which one is most reactive towards `S_(N)1` reactions ?

Options

A. `C_(6)H_(5)C(CH_(3))(C_(6)H_(5))Br`
B. `C_(6)H_(5)CH_(2)Br`
C. `C_(6)H_(5)CH(C_(6)H_(5))Br`
D. `C_(6)H_(5)CH(CH_(3))Br`.

Answer

A

Explanation

For `S_(N)1` reactions, more stable is the carbocation, more reactive the alkyl or aryl halide. The stability of carbocations increases in the order :
`C_(6)H_(5) overset(+)CH_(2) lt C_(6)H_(5)overset(+)CHCH_(3) lt C_(6)H_(5)overset(+)CHC_(6)H_(5) lt C_(6)H_(6)overset(+)C(CH_(3))C_(6)H_(5)`
Since (a) forms the most stable carbocation, it is most reactive.
`underset("(Most reactive)")(C_(6)H_(5)-underset(CH_(3))underset(|)overset(C_(6)H_(5))overset(|)C-Br) overset(-Br^(-))to underset("(Most stable carbocation)")(C_(6)H_(5)-underset(CH_(3))underset(|)overset(C_(6)H_(5))overset(|)(C^(ox)))`